1. Field of the Disclosure
The disclosure relates to an optical control apparatus that controls intensities, wavelengths, phases, polarization states, paths, or the like of light.
2. Description of the Related Art
Modes of recent optical communication systems are developing from point-to-point type networks to ring type or mesh type networks. Nodes of networks of such modes require optical switch apparatuses, which are optical control apparatuses for inputting and outputting arbitrary signal light to and from arbitrary ports and arbitrarily changing paths of the signal light. Particularly, when wavelength multiplexing signal light obtained by wavelength division multiplexing of signal light beams having wavelengths different from one another is used, a wavelength selecting optical switch apparatus, which is able to arbitrarily change a path of signal light having an arbitrary wavelength, is required.
Such an optical switch apparatus uses liquid crystal on silicon (LCOS) to switch a path of signal light (see U.S. Patent Application Publication Nos. 2006/0067611 and 2005/0276537). LCOS is a spatial optical modulator that is able to modulate a phase of input light by liquid crystal and to diffract the light. In the optical switch apparatus using LCOS, optical switch operations are realized by diffracting signal light input from a certain path by LCOS and outputting the signal light to a specific path.
LCOS has polarization dependent characteristics because birefringence of liquid crystal is used. The optical switch apparatus using LCOS includes a polarization separation element and a polarization rotation element in order to solve this problem. Such an optical switch apparatus is configured such that a polarization separation element separates signal light input to the optical switch apparatus into two signal light beams of linear polarizations orthogonal to each other, and a polarization rotation element rotates a polarization direction of one of the signal beams to match a polarization direction of the other one of the signal light beams so that the two signal beams with their polarization directions coinciding are incident on LCOS. As a result, the problem of the polarization dependent characteristics is solved because the signal beams having only the single polarization direction are incident on LCOS.